A luminous pigment means a pigment having the ability to keep luminescence in a high luminance for a given length of time even in case of keeping it in a dark place after irradiation with light, and is also called a light-storage pigment.
Conventionally, various kinds of pigments are known for such luminous pigments in which a metal sulfide such as calcium sulfide, zinc sulfide, cadmium sulfide or the like is doped with copper, bismuth or the like. However a duration time of luminescence in a dark place is short, and these pigments cannot help being used under an extremely restricted condition.
To improve this point attempts have been made in which various radioactive substances are added to the above compounds, but it was extremely restricted from the safety point of view, because the effect to a human body due to the use of the radioactive substances cannot be neglected.
Consequently, as those having no such defects, and having a long luminous time in a dark place and further no safety problem, luminous pigments consisting of compounds, which are made of some kind of metal oxides doped with rare-earth metal elements, were developed (JP, A, H9-143463, Japanese Patent No. 2543825, Japanese Patent No. 2697733, Japanese Patent No. 2697688).
However, it was found that such luminous pigments have a fatal disadvantage in that they are easily decomposed in the presence of water. This problem cannot be solved by merely changing the type of a vehicle component. These pigments lose a luminous effect by moisture adsorption, condensation or the like regardless of an outdoor or indoor use, and it was a fact that said pigments could not practically be used while retaining their properties.
Recently, water resistant pigments were thus developed (JP, A, H10-273654, Japanese Patent No. 2929162). Especially, the patented invention of the Japanese Patent No. 2929162 by the present inventors, relates to water resistant luminous pigments, in which a metal oxide is doped with a rare-earth metal element, and then treated by at least one compound of acids and acid generating substances.
However, it has been found that it lost water resistance when used at a high temperature for a long period of time, though there is no particular problem in its use under ordinary temperature, and thus, it became necessary to undertake the solution.